LoongArch: KVM: Add EIOINTC user mode read and write functions

Implement the communication interface between the user mode programs
and the kernel in EIOINTC interrupt controller simulation, which is
used to obtain or send the simulation data of the interrupt controller
in the user mode process, and is also used in VM migration or VM saving
and restoration.

Signed-off-by: Xianglai Li <lixianglai@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>